I've been using Red Brand mediums on my D-18 for a while now. When I put on my first set, I was so fascinated by the tone that I felt like I did as a teenager playing my first few chords on my new guitar ~ the sound was just so great, so deep and resonant, that I felt like I was playing a new, better instrument.

If you prefer phosphor bronze to "regular" bronze, you'll probably like the Reds:

Per Wikipedia: "Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive, but sometimes with other elements such as phosphorus, manganese, aluminium, or silicon."

Traditional "80/20" bronze strings are 80% copper; phosphor bronze is about 92% copper, and the new Red strings are 99% copper. I've asked what the other 1% might be, and they're not saying; might be a trade secret. Whatever it is, it prevents the copper from oxydizing and turning green like the Statue of Liberty.

I've been fortunate enough to receive a number of free sets from Red via two different promotions*. Good thing; they're not cheap: 12 bucks by mail order. Along with several sets of free-sample Reds in my preferred weight/guage (medium/13), they also sent one set of their other product, Cleartone phosphor bronze, which are supposedly similar to coated Elixir-type strings in longevity without the slick feeling. After playing on Reds for a while, the phosphor bronzes sounded thinner and "tinnier" than what I had become used to. (Makes sense, since there's more tin alloyed with the copper.)

"MMMV," of course (Your mileage may vary).

Whether you like the Red Brand strings or not, I think we can all agree that medium rather than light weight strings are the best choice with Martin dreadnaughts. Well, most of us ~ "all" of any group never agrees on anything!

*The free-sample promotions have been publicized on the company's facebook page; I'd recommend you check that out if you can. If not, try www.redstrings.com.

An interesting pro-and-con discussion on the Acoustic Guitar forum:
http://www.acousticguitar.com/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=1;t=029275